The small leucine rich repeat proteoglycans (or SLRPs) are a group of extracellular proteins (ECM) that belong to the leucine-rich repeat (LRR) superfamily of proteins (Iozzo, R. V., and Murdoch, A. D. (1996) Faseb J. 10(5), 598-614; Hocking, A., et al. (1998) Matrix Biology 17, 1-19). The LRR is a protein folding motif composed of 20-30 amino acids with leucines in conserved positions. LRR-containing proteins are present in a broad spectrum of organisms and possess diverse cellular functions and localization (Kobe, B., and Deisenhofer, J. (1994) Trends in Biochemical Sciences, 415-421). The members of the SLRP subfamily have core proteins of similar size (about 40 kilodaltons) that are dominated by a central domain composed of 6-10 tandemly repeated LRRs. This domain is flanked by smaller, less conserved N-terminal and C-terminal regions containing cysteines in characteristic positions.
Most of the SLRP proteins are proteoglycans, and the SLRP gene family has been subdivided into 3 classes based on similarities in overall amino acid sequence, spacing of cysteine residues in the N-terminus, and gene structure. The previously identified class I members, decorin (Krusius, T., and Ruoslahti, E. (1986) Proc. Natl. Acad. Sci. U.S.A. 83(20), 7683-7) and biglycan (Fisher, L. W., et al. (1989) J. Biol. Chem. 264(8), 4571-4576), are the closest related SLRPs based on amino acid sequences; the human sequences are 57% identical. The N-terminal regions of decorin and biglycan are substituted with one and two chondroitin/dermatan-sulfate chains, respectively. The cysteine-rich cluster in the N-terminus of class I SLRPs has an amino acid spacing of CX3CXCX6C (SEQ ID NO:23). The mouse decorin (Scholzen, T., et al. (1994) J. Biol. Chem. 269(45), 28270-81) and biglycan genes (Wegrowski, Y., et al. (1995) Genomics 30(1), 8-17) contain 8 exons.
Class II members, fibromodulin (Oldberg, A., et al. (1989) EMBO 8(9), 2601-2604), lumican (Blochberger, T., et al. (1992) J. Chem. 267(1), 347-352), PRELP (Bengtsson, E., et al. (1995) J. Biol. Chem. 270(43), 25639-25644), keratocan (Corpuz, L. M., et al. (1996) J. Biol. Chem. 271(16), 9759-63), and osteoadherin (Sommarin, Y., et al. (1998) J. Biol. Chem. 273(27), 16723-9), have a pairwise amino acid sequence identity ranging between 37-55% and have a common gene structure composed of three exons. The cysteine spacing in the N-terminal region of class II SLRPs is identical (CX3CXCX9C (SEQ ID NO:24)) but different from the other SLRP classes. The core proteins of class II SLRPs (with the exception of PRFLP) can be substituted with N-linked keratan sulfate glycosaminoglycan chain(s).
The class III members, epiphycan/PG-Lb (Shinomura, T., and Kimata, K. (1992) J. Biol. Chem. 267(2), 1265-1270; Johnson, H. J., et al. (1997) J. Biol. Chem. 272, 18709-18717), osteoglycin/mimecan (Madisen, L., et al. (1990) DNA Cell. Biol. 9(5), 303-309; Funderburgh, J. L., et al. (1997) J. Biol. Chem. 272(44), 28089-28095), and opticin (Reardon, A. J., et al. (2000) J. Biol. Chem. 275(3), 2123-2129), have a pairwise amino acid sequence identity ranging between 35-42% and have a common gene structure composed of either 7 or 8 exons. Class III SLRPs contain only 6 LRRs, and the cysteine spacing in the N-terminal region of class III SLRPs is unique (CX2CXCX6C (SEQ ID NO:25)). The recently identified opticin is substituted with O-linked sialylated oligosaccharides, and consequently is a glycoprotein rather than a proteoglycan. On the other hand, osteoglycin/mimecan and epiphycan can be substituted with N-linked keratan sulfate glycosaminoglycan chain(s) and O-linked chondroitin/dermatan sulfate chain(s), respectively. Interestingly, many of the SLRP proteoglycans have been isolated without attached glycosaminoglycans, suggesting that they are “part-time” proteoglycans (Grover, J., et al. (1995) J. Biol. Chem. 270(37), 21942-21949; Corpuz, L. M., et al. (1996) J. Biol. Chem. 271(16), 9759-9763; Funderburgh, J. L., et al. (1997) J. Biol. Chem. 272(44), 28089-28095).
Several SLRP proteins display potent effects in vitro. For example, recombinant decorin, biglycan, and fibromodulin bind to TGF-β in vitro (Hildebrand, A., et al. (1994) Biochem. J. 302, 527-534), and decorin can interfere with TGF-β dependent proliferation of Chinese hamster ovary (CHO) cells (Yamaguchi, Y., and Ruoslahti, E. (1988) Nature 336(6196), 244-246). Furthermore, injection of decorin into rats with experimental glomerulonephritis curtailed the abnormal deposition of matrix suggesting that decorin may affect TGF-β activity also in vivo (Border, W. A., and Ruoslahti, E. (1990) Cell Differ. Dev. 32(3), 425-431; Border, W. A., et al. (1992) Nature 360(6402), 361-364). Recently, it has been shown that decorin can down-regulate epidermal growth factor receptor (EGFR) leading to growth suppression, and decorin may act as a natural inhibitor of the EGFR signaling pathway (Csordas, G., et al. (2000) J. Biol. Chem. 275(42), 32879-32887).
The SLRPs have been shown to interact with a variety of extracellular matrix proteins, such as collagens (Gallagher, J. T., et al. (1983) Biochem. J. 215(1), 107-116), fibronectin (Schmidt, G., et al. (1987) J. Cell. Biol. 104(6), 1683-1691), and thrombospondin (Winnemoller, M., et al. (1992) Eur. J. Cell. Biol. 59(1), 47-55), as well as serum proteins, heparin cofactor II (Whinna, H. C., et al. (1993) J. Biol. Chem. 268(6), 3920-3924) and C1q (Krumdieck, R., et al. (1992) J. Immunol. 149(11), 3695-3701). Biochemical assays have demonstrated that decorin (Vogel, K. G., et al. (1984) Biochem. J. 223(3), 587-597), fibromodulin (Hedbom, E., and Heinegard, D. (1989) J. Biol. Chem. 264(12), 6898-6905), and lumican (Rada, J. A., et al. (1993) Exp. Eye Res. 56(6), 635-648) bind to collagens in vitro and modulate collagen fibril formation. Morphological analysis of mice “knockouts” demonstrates that decorin (Danielson, K. G., et al. (1997) J. Cell Biol. 136, 729-743), fibromodulin (Svensson, L., et al. (1999) J. Biol. Chem. 274(14), 9636-9647), and lumican (Chakravarti, S., et al. (1998) J. Cell. Biol. 141(5), 1277-1286), respectively, are necessary for normal collagen fibril formation in specialized connective tissues of skin, tendon, and cornea. Therefore, a role for SLRPs in collagen fiber formation is clearly established both in vivo and in vitro. Also, biglycan-null mice exhibit a mild osteoporosis-like phenotype (Xu, T., et al. (1998) Nat. Genet. 20(1), 78-82). Recently, patients with cornea plana 2 (CNA2; MIM 217300) were shown to have mutations in the keratocan gene, a class II SLRP family member (Pellegata, N. S., et al. (2000) Nat. Genet. 25(1), 91-95).
Nucleotide sequencing of a human bacterial artificial chromosome (BAC, RPC111-91705), and contigs of overlapping BAC clones revealed that four SLRPs genes (decorin, lumican, keratocan, and epiphycan/PG-Lb) are physically linked on human chromosome 12q (Pellegata, N. S., et al. (2000) Nat. Genet. 25(1), 91-95). Previous genetic linkage studies in the mouse suggested that decorin, lumican, and epiphycan map together in a cluster in close proximity to the MgfX gene on mouse chromosome 10, and these genes are deleted in mice that have large deletion mutations at the Steel locus (Danielson, K. G., et al. (1999) Mamm. Genome 10(2), 201-203).
Despite the research performed to date, there still exists a need for an increased understanding of the molecular structure and function of the SLRP proteins. This understanding can be approached by further studies of the proteins themselves, their encoding nucleic acid sequences, and their interactions with other proteins and biological compounds in cellular systems.